Technical Field
The disclosure relates to the field of manufacturing a polycrystalline dielectric layer on metal and, more especially, manufacturing a dielectric capacitor layer inserted between two metal electrodes.
Description of the Related Art
Dielectric layers that have a high dielectric constant have many applications, especially in compact, high-capacitance capacitors.
The dielectric constant of a dielectric material depends not only on the atoms of which it is made but also on the spatial arrangement of the molecules relative to each other. For example, zirconium oxide (ZrO2) has a dielectric constant equal to 18 in its amorphous form, i.e. a form that has no particular structure, and a dielectric constant of 45 when it has a tetragonal or cubic crystalline structure.
One way of increasing the capacitance of a capacitor without increasing the volume of its dielectric layer is therefore to produce the latter in a crystalline form.
The form and final geometry of a crystalline structure depend on the crystal germination conditions and hence on the geometrical and chemical structure of the surface on which it is manufactured. However, crystalline growth of a dielectric material is tricky when growth takes place on metal. In fact, the metal may induce a “polycrystalline” form of the dielectric material, i.e. a dielectric material that is formed by juxtaposed crystals. The interface between these crystals is usually referred to as the “grain boundary”.
The term “polycrystalline” denotes a material comprising crystals that may be embedded in a matrix of dielectric material that has not crystallized, i.e. an amorphous dielectric material. There are therefore different crystallization rates depending on the total volume of crystals, given that the crystallization rate is defined as the ratio of the volume of crystals to the total volume of material. The dielectric constant of a dielectric material increases as its crystallization rate increases. In practice, a dielectric layer that has the highest possible degree of crystallization is therefore sought after.
Since a polycrystalline form implies the presence of grain boundaries, grain boundaries that extend through the entire thickness of the dielectric layer are very frequently observed. Such grain boundaries, referred to as “penetrating grain boundaries” in the rest of this document, constitute preferential leakage paths for electrons and this is extremely detrimental. In the case of a capacitor, in particular, this means that penetrating grain boundaries electrically connect the two electrodes of the capacitor. In addition, it has also been observed that the existence of a penetrating grain boundary substantially reduces the latter's electrical breakdown voltage.
In order to prevent the occurrence of penetrating grain boundaries, an interlayer made of an amorphous dielectric material is provided in the median plane of the dielectric layer. For example, a dielectric layer with no penetrating grain boundaries comprises a stack formed by a layer of amorphous alumina (Al2O3) placed between two layers of polycrystalline ZrO2. Although the alumina layer prevents the occurrence of penetrating grain boundaries, it nevertheless significantly limits polycrystalline growth of the ZrO2, thereby lowering the final crystallization rate of the ZrO2 layers and consequently also reducing the total dielectric constant of the dielectric layer. Thus, with such a structure, the maximum dielectric constant that can be achieved is 20.
Inserting an amorphous dielectric layer in order to prevent the occurrence of penetrating grain boundaries therefore defeats the first object referred to above, namely obtaining a polycrystalline structure that has the highest possible dielectric constant.